The invention relates to a process for the regulation of the aeration of a biological wastewater treatment plant for the removal of carbon and nitrogen pollution.
It is known that the purification of wastewater constitutes a major problem. Accordingly, the European Union has been led to issue a directive (No. 91/271/ EEC) relating to the treatment of urban wastewater which determines the limits of discharges, into the natural environment, of untreated wastewater. Thus, each treatment unit is attributed a precise objective as regards the quality of the water treatment, it being possible for failure to achieve such an objective to give rise to penalties of a financial or even a penal nature.
The majority of urban wastewater treatment plants use the activated sludge process. An important phase of this process consists in the removal of the carbon and the nitrogen contained in the wastewater, by sequencing of the periods of aeration. It is indeed known that the main problem encountered in wastewater treatment plants is adapting the treatment to the variations in the rate of entry of the water to be purified and to its polluting load, so as to obtain a constant quality of purified water and the minimum regulatory quantity of polluting discharges into the natural environment. For this purpose, the removal of carbon and nitrogen requires a very strict and precise control of aeration given that this removal must correspond to two requirements. According to the first, a sufficient total duration of aeration should be provided per day in order to carry out the oxidation of the carbon components of the wastewater and the stabilization of the sludge; the second is linked more directly to the daily distribution of the aeration phases in order to successfully carry out the removal of the nitrogen. On the one hand, it is necessary to observe a sufficient period for maintaining under aerobic conditions for the sludge to perform the nitrification and, on the other hand, the denitrification requires an appropriate effluent residence time under anoxic conditions. For this purpose, in the small-load activated sludge processes used in a single aeration basin, the removal of nitrogen compounds results from a strict control of the alternation of the aerated and nonaerated sequences.
Any defect in the setting or the operation of the oxygen supply devices results in a malfunction of the wastewater purification stations, with repercussions on the quality of the effluent treated, the equilibrium of the purifying biomass and the characteristics of the sludge produced.
A lack of adaptation of the aeration sequences therefore has effects in the short term on the quality of the water obtained which may then contain nonoxidized nitrogen compounds if the periods of aeration are not sufficiently long, or nitrates if the periods of anoxia are too short. By contrast, when the periods of nonaeration are too long, the effluent to be treated encounters anaerobic conditions which must be absolutely avoided. Indeed, the phenomena of anaerobiosis in the treatment basin, linked to an under-oxygenation of certain zones, cause in the long term the appearance of filamentous bacteria and these microorganisms induce a modification of the structure of the floc and a reduction in its sedimentation ability, which of course has an unfavourable repercussion on the quality and the cost of the treatment. Another consequence of an insufficient cumulative duration of aeration relates to the quality of the sludge and, in particular, determines its stability.
It can be understood why the regulation of aeration is one of the key points in such a water treatment process. Various methods of regulation have been used. Among these, the techniques below may be mentioned:
1) The most rustic technique in the field of the aeration of biological basins is the timer which, depending on a program defined by the user, makes it possible to deliver oxygen at defined times of the day, without any correlation with the real need of the plant,
2) a slightly improved technique is the decision for aeration based on a high threshold and a low threshold of the oxidation-reduction potential (Redox potential) measured on the plant: the decision is taken on information at the time T, information which can be refuted a few seconds later,
3) a more rigorous technique has been developed by the present holder (FR-A-2,724,646). This method takes into account various Redox potentials, the derivative of the variation of this Redox potential and the history of the purification station, thus approaching the notion of an expert system,
4) other regulation logics are based on the measurement of the concentration of oxygen in the water to be purified; given that this notion does not really make sense, during the phases for nonaeration of the basins, the measurement of the oxygen concentration is then replaced with a timer which will assign a nonaeration time (period of anoxia) for example from 40 to 60 minutes, followed by a period during which the regulation would be effectively performed on the reference value for oxygen, with for example gradual stopping of the aeration turbines, one by one,
5) some attempts at regulation were made with respect to the measurement of the concentration of the ammoniacal and nitro compounds in the aeration basin, by characterizing the nitrification and denitrification efficiency and, subsequently, the oxygen requirements of the purification station.
The use of the various prior art techniques mentioned above reveals numerous disadvantages which demonstrate their limitations. Among these disadvantages, the following may be mentioned in particular:
1) automatic control by a timer obviously takes no account of the variations in polluting load imposed on the station by the irregular nature of the supply of water to be treated.
2) the method of Redox thresholds does not take into account the history of the purification station, such as temporary overloads, temporary breakdown of equipment, and the like.
3) the process according to FR-A-2,724,464 makes it possible to provide efficient depollution, on the carbon pollution, at the nitrification level and at the denitrification level. However, the objective of this method is to provide sufficient aeration which in fact always results in an excess supply of oxygen. However, this excess can be a drawback from an economic point of view. Furthermore, from the point of view of the water treatment, the aeration phase is affected because it starts in the presence of a relatively high content of oxygen, from 7 to 8 mg/l, which has to be removed before starting to use the oxygen of the nitrates. For the same period of denitrification, the phase for stopping the aeration will be correspondingly longer, and the effective treatment time is reduced over a day.
4) the use of the technique based on a simple measurement of the concentration of oxygen in the water to be purified does not make it possible to control the state of progress of the denitrification reactions which occur in the activated sludge basins since there is a need for a zero oxygen concentration to accomplish this phase. Furthermore, the oxygen concentrations necessary to bring about the nitrification can change in a range going from 3 to 7 mg/l, depending in particular on the state of oxidation of the sludge present in the biological reactor.
5) the technique based on the measurement of the concentration of the ammoniacal and the nitro compounds in the aeration basin can only be envisaged in large stations, given the high cost of the equipment. Moreover, this technique takes no account of the oxidation state of the sludge whereas a measurement of the oxidation-reduction potential according to FR-A-2,724,646 makes it possible to evaluate the physiological state of the sludge.
The disadvantages of the abovementioned prior art solutions therefore lead to a search for means which make it possible to optimize the various reactions in the aeration basins of the purification station, while controlling more judiciously the supply of oxygen to the latter. This constitutes the problem solved by the present invention.
The subject of the present invention is therefore a process for regulating the aeration in a biological wastewater treatment plant using a carbon removal stage, a nitrification stage and a denitrification stage, characterized in that it comprises the following stages:
1) continuously measuring the value of the oxidation-reduction potential of the treated medium, making it possible to activate or otherwise the aeration means;
2) during the aeration phases, measuring the oxygen concentration and exploiting its value, together with those of the oxidation-reduction potential in order to:
maintain the aeration if the oxygen concentration corresponds to a reference range;
reduce the aeration if the oxygen concentration is above the reference range, and
increase the aeration if the oxygen concentration is below the reference range, and
3) carrying out a self-adjustment of the reference values for oxygen by comparing, during the transition of the oxidation-reduction potential xe2x80x9cpartial nitrification/total nitrificationxe2x80x9d, the real oxygen concentration with the reference range for the system and adjusting the reference value according to the real oxygen need of said system.
According to the present invention, when the Redox transition xe2x80x9cpartial nitrification/total nitrificationxe2x80x9d exists in the lower region of the reference range for the oxygen concentration, the value of this reference is reduced.
According to the invention, when the value of the Redox transition xe2x80x9cpartial nitrification/total nitrificationxe2x80x9d is not reached for the reference value for the oxygen concentration, the latter is increased.
It is evident why the process which is the subject of the present invention constitutes a combination of a process as defined in FR-A-2,724,646, comprising a monitoring of the derivative of the variation of the oxidation-reduction potential, with a monitoring of the xe2x80x9coxygen concentrationxe2x80x9d signal solely during the aeration phase. Schematically, the analysis of the oxidation-reduction potential values makes it possible to define the duration of the start and stop sequences for the plant and the analysis of the oxygen concentration values makes it possible to control the power which has to be used during the aeration phases.
According to the invention, there is added to this combination measurement of the Redox potential+measurement of the oxidation concentration as absolute value [O2], the continuous comparison of said parameters which makes it possible to deduce and to confirm the coherence of the information gathered on the station. Thus, by virtue of the use of the process according to the invention, various information and possibilities for crucial interventions relating to the state of the station are accessible such as, in particular:
the control of the reliability of the sensors for the Redox potential and for the oxygen concentration;
the control of the reliability of the equipment or the identification of the pollution overloads;
the finer analysis of the biological state of the sludge,
the possibility of readjusting the reference values for the oxygen concentration as a function of the performance of the sludge, and
embarking on the denitrification phase with a relatively low content of residual oxygen and ensuring a rapid initiation of the denitrification phase.